Abstract

Mesozoic and Tertiary faulting within a 280-km-long, west-northwest–trending zone in Coahuila, Mexico, is inferred from stratigraphic and structural evidence. The fault forms the northern boundary of the Late Jurassic–Early Cretaceous Coahuila Island. Uplift on the south during the Jurassic and Early Cretaceous is indicated by a large detrital lithosome that formed on the northern side of the fault. Clast size within the Early Cretaceous part diminishes northward where the unit is divided into upper and lower parts by a limestone tongue. These two Early Cretaceous detrital units plus two Jurassic conglomerates at Valle San Marcos suggest episodic, pre-Tertiary movement. Deformation of 242 ± 2-m.y.-old granites at Valle San Marcos also reflects several periods of pre-Tertiary movement. Uplift on the north along the same fault zone during the Tertiary (Laramide) raised Permian and Jurassic rocks into juxtaposition with Cretaceous limestones. Extensive left-lateral movement is neither proven nor precluded. Nevertheless, because of the size, orientation, general location, and times of movement of the fault, we suggest that it may either be part of the megashear postulated by several authors for northern Mexico, or a splay of that megashear.